1. Field of the Invention
The present invention relates to a temperature estimating device configured to estimate the temperature of an electric motor.
2. Description of the Related Art
According to a known method of detecting or estimating the temperature of an electric motor, loss is estimated from current supplied to the electric motor and the loss obtained by this estimate is used as the basis to calculate the temperature of the electric motor. According to another known temperature estimation method, a temperature sensor provided inside of the electric motor is utilized. A temperature sensor is generally attached near a main heat generating source of the electric motor, that is, a winding. Due to this, it is possible to monitor the state of heat generation in the electric motor.
FIG. 8 shows an example of the configuration of a temperature sensor 100. The temperature sensor 100 is provided with a temperature detecting element 102 and a lead 104 connected to the temperature detecting element 102. The temperature detecting element 102 is covered by a resin 106 so as to improve insulating ability and heat resistance. In the case of such a temperature sensor 100, since the heat capacity of the resin 106 is relatively large, the heat generated from the winding is sometimes not transmitted to the temperature detecting element 102 sufficiently fast. For this reason, the detected temperature detected by the temperature sensor 100 cannot keep up with the change of the actual temperature of the electric motor (winding) and as a result the temperature of the electric motor can no longer be accurately detected. This is particularly remarkable when the amount of temperature change of the winding with respect to time is large.
FIG. 9 shows the detected temperature obtained by the temperature sensor 100 and the actual temperature of the winding. The solid line in the figure shows the detected temperature, while the dashed line shows the winding temperature. As shown in the figure, in the range of the region X, the amount of change of the winding temperature with respect to time is relatively large. Therefore, it can be understood that the difference between the detected temperature and the winding temperature becomes larger. As opposed to this, in the range of the region Y, the amount of change of the winding temperature with respect to time is relatively small and thus the difference of the detected temperature and the winding temperature is small.
With the method of estimating the temperature of an electric motor from the loss occurring in the electric motor, the configuration required for estimating the temperature is complicated. On the other hand, with the method of using a temperature sensor, as explained above, it is difficult to accurately detect the temperature of an electric motor.
Japanese Patent Publication No. S62-082692A discloses an induction heating cooker wherein an amount of power supplied to a heating winding is controlled in accordance with a rate of change of detected temperature with respect to time for the purpose of preventing overheating in such a cooker. However, Japanese Patent Publication No. S62-082692A does not disclose estimating the temperature of an object to be measured.
Japanese Patent Publication No. S62-203578A discloses a method of controlling startup of an induction electric motor configured so as to predict a temperature change of a winding and to warn an operator and restrict startup of the electric motor when it is predicted that the winding temperature will reach a predetermined temperature or more. However, the invention described in Japanese Patent Publication No. S62-203578A predicts the winding temperature during operation from the winding temperature detected before startup of the electric motor and cannot accurately detect the actual temperature of the electric motor.
Therefore, there is a need for a temperature estimating device which can accurately estimate the temperature of an electric motor by a simple method.